File:A lichen - Tephromela atra ("Black Shields") - geograph.org.uk - 1057655.jpg

English: A lichen - Tephromela atra ("Black Shields") (Older synonym: Lecanora atra)

This lichen is "common on siliceous rocks and walls, rarely on trees, particularly characteristic on rocks by the sea above the high-tide mark" [Roger Phillips, "Grasses, Ferns, Mosses and Lichens of Great Britain and Ireland"]. This is one of very many examples of this species that were growing on rocks on the upper part of the beach. The photograph shows a mature specimen.

This lichen often occurs "with a thin dark prothallus" [F.S.Dobson, "Lichens - An Illustrated Guide to the British and Irish Species"]; this is an area at the margin of the lichen which lacks algal cells (such a prothallus is visible in this photograph). The same work notes regarding the apothecia (the spore-producing discs) that these are "large, up to 3mm diameter .... When mature the disc is flat with a grey contorted and crenulate margin".

[This species is similar to Lecanora gangaleoides, whose discs are persistently convex and have less contorted margins; also, if one of the discs is sliced open, T. atra shows an area of violet-brown colour inside, while L. gangaleoides shows a greenish-brown colour.]

As for the lichen's common name, Black Shields, "The Oxford Book of Flowerless Plants" (Nicholson/Brightman) states that eighteenth-century naturalists used to refer to the discs (apothecia) as "shields".